In recent years, size and weight reduction of electrical appliances has propelled the development of lithium secondary cells with high energy density. Further improvements in cell property have been required with extension of the application fields of such secondary cells.
Under such circumstances, secondary cells containing metallic lithium as the negative electrode, have intensively studied as candidates capable of achieving high capacities. However, during repeated charge and discharge cycles, metallic lithium grows into a dendritic crystal. When the dendritic crystal reaches the positive electrode, short circuiting occurs in the cell. This is the most serious obstacle inputting lithium secondary cells with metallic lithium negative electrodes to practical use.
There have also been provided nonaqueous electrolyte secondary cells containing carbonous materials as negative electrodes, which are capable of occluding and discharging lithium, such as cokes, artificial graphite, or natural graphite, in place of metallic lithium. Such secondary cells can exhibit improved service life and safety since lithium does not grow into the dendritic crystal. In particular, nonaqueous electrolyte secondary cells made from graphite-based carbonous materials, such as artificial graphite and natural graphite, have attracted attention as ones capable of meeting the need for high capacities.
Further, negative-electrode active materials made from alloys such as, for example, silicon (Si), tin (Sn), and lead (Pb) have lately been proposed in order to achieve higher capacities (see, for example, Patent Documents 1 and 2).
Furthermore, electrolytic solutions containing different compounds in addition to electrolytes and main solvents have been proposed in order to enhance properties of nonaqueous electrolyte secondary cells, such as load, cycle, storage, and low-temperature characteristics.
For example, electrolytic solutions containing carbonate derivatives having unsaturated bonds have been provided, such as ones containing certain amounts of vinylene carbonate and its derivatives in order to inhibit decomposition of the electrolytic solutions for nonaqueous electrolyte secondary cells including graphite negative electrodes (see, for example, Patent Document 3), and ones containing certain amounts of ethylene carbonate derivatives having a non-conjugated, unsaturated bond at the side chain thereof (see, for example, Patent Document 4).
For electrolytic solutions containing such compounds, the compounds are reductively decomposed on the surfaces of the negative electrodes to form films thereon, and the films inhibit extra decomposition of the electrolytic solutions. Electrolytic solutions containing halogen-containing carbonates have also been proposed (see, for example, Patent Document 5).
Patent Document 6 describes that an electrolytic solutions containing a single solvent, i.e., fluoromethyl ethylene carbonate (4-(monofluoromethyl)-1,3-dioxolan-2-one) in combination with various PF6 and BF4 salts, is excellent in oxidation resistance compared to propylene carbonate or trifluoropropylene carbonate, and thus is useful as an electrolytic solution for a variety of devices. This document also describes that lithium secondary cells containing the electrolytic solution exhibit excellent cycle characteristics and that electric double-layer capacitors containing the electrolytic solution excel in continuous application characteristics at elevated temperatures.
Patent Document 7 describes that electrolytic solutions which contain 60% by volume or more of fluoromethyl ethylene carbonate in their solvents, or difluoromethyl ethylene carbonate and contain (C2H5)4NBF4 or (C2H5)4NPF6 as a supporting electrolyte are excellent in oxidation resistance, and thus is useful as electrolytic solutions for electric double-layer capacitors.
Linear carbonate compounds have generally been used as materials for polycarbonates, and as materials and diluent solvents for medicines and agrichemicals. These have also been used in a broad range of applications, including electrolyte solvents and solid electrolytes for energy storage devices such as Li cells, electrolytic capacitors, and electric double-layer capacitors; and electrode compositions (see, for example, Patent Documents 8 and 9).
For currently used carbonate compounds, further improved performances and added functionalities are still needed because of improvements in synthetic and purification processes or development of advanced processes in the fine chemical field; technological innovation in the fields of agrichemicals and fertilizers; and needs for highly functionalized semiconductors and energy devices subsequent to the downsizing and densifying in the field of electrical and electronic equipment.
[Patent Document 1] Japanese Patent Application Laid-open No. HEI 11-176470
[Patent Document 2] Japanese Patent Application Laid-open No. 2004-87284
[Patent Document 3] Japanese Patent Application Laid-open No. HEI 8-45545
[Patent Document 4] Japanese Patent Application Laid-open No. 2000-40526
[Patent Document 5] Japanese Patent Application Laid-open No. HEI 11-195429
[Patent Document 6] Japanese Patent Application Laid-open No. HEI 9-251861
[Patent Document 7] Japanese Patent Application Laid-open No. HEI 10-233345
[Patent Document 8] Japanese Patent Application Laid-open No. HEI 1-311054
[Patent Document 9] Japanese Patent Application Laid-open No. SHO 61-64082